Methods for sorting articles have become increasingly reliant upon the use of bar codes and other similar data forms for making rapid identification of items. Many of these marking systems rely upon coding that can be “read” by an electronic system. Such systems typically require illumination of the marking, optical imaging and signal processing to ascertain information carried by the marking. Advantages of such systems include offering users an ability to automate identification steps of various processes. However, certain situations can render the use of existing technology ineffective. As an example, a summary of mail sorting techniques provides an example of the challenges faced by individuals reliant upon existing technology for identification and sorting of items.
The United States Postal Service (USPS) currently sorts mail using a bar code system. In order to sort the mail, the USPS optically reads address information with an optical character recognition (OCR) imaging system. A bar code is then applied by the USPS to the mail piece, which provides for subsequent identification and sorting prior to delivery. This type of mail sorting technique is described in European Patent EP 509280-A2, entitled “Bar code translation for deferred optical character recognition mail processing—allowing use of local formats of bar code reading and sorting of mail pieces during incoming sort.” For most mail pieces, reading the bar code is not a problem, as white or light colored backgrounds provide adequate contrast, thus allowing bar code imaging equipment to operate effectively.
However, it has been discovered that problems arise when colored, multi-colored or complex backgrounds lie beneath the bar code. In such instances, the nature of the substrate background typically dampens the signal to noise ratio (SNR) in the bar code imaging equipment, or otherwise causes problems, thus providing incorrect or incomplete information to system operators and/or to automated equipment that relies on a correctly read bar code. The reduced reliability in the imaging of coded information lying on top of the substrate background typically results from poor contrast between the coded information and the substrate background.
For example, business mail and periodicals often contain multi-colored graphical patterns associated with decorative elements and advertising on outer surfaces of the mail piece, and the mail piece itself may be enclosed within a transparent plastic wrapping. If a bar code or some other computer readable indicia is to be applied to and then read from the mail piece, or the plastic wrapping, then it can be realized that the underlying graphical pattern can significantly interfere with the ability of a bar code scanner mechanism to correctly read the bar code.
While at first glance it might appear that one could simply apply a neutral label to the mail piece, and then place the bar code on the label, this approach would be objectionable for a number of reasons. First, it adds cost and complexity to the mail piece marking and coding process. Second, the label might be applied over an important element of the underlying graphical pattern, such as over a telephone number or over an Internet address of a company that has placed an advertisement on the outer surface of the mail piece. Third, the presence of the label may be visually and aesthetically objectionable when located upon a carefully designed artwork pattern that forms a portion of an advertisement or some other type of message or decoration on the mail piece.
Thus, a method of solving these problems that involves the application of a contrasting label that carries bar code information is problematic, as application of a separate label may obscure important information on the mail piece and/or it may cause other problems.
There exist numerous bar code applications where the appearance of a standard black and white bar code is unacceptable. Various invisible marking schemes, some of which are represented in U.S. Pat. Nos. 5,093,147, 5,282,894, 5,423,432, 5,614,008, 5,684,069, 5,686,725, 5,703,229, 6,149,719 attempt to avoid the use of the standard black and white bar code. However, the difficulty of incorporating these schemes is often increased when the background has variable colors or markings. In general, a colored background has a spatially variable reflectivity which can serve to greatly effect the contrast of invisible markings that are presented as an alphanumeric or bar code.
Several solutions have been presented to compensate for a non-uniform background. During the early development of the fluorescent bar coding scheme used on U.S. mail, it was suggested that the color of the background could be measured in order to change the amount of material that was printed for the bar code. White envelopes of a high reflectivity would be printed with less material than manila envelopes which required more material to compensate for the lower reflection from the substrate towards the bar code reader. This approach is successful for substrates which are uniform in color, but does not solve the problem of a varied background caused by writing or printing beneath the bar code.
U.S. Pat. No. 4,983,817 attempts to solve this problem by measuring both the returned probe beam and fluorescence intensity. Since the wavelength of the probe beam is spectrally close to the returned fluorescence wavelength, it can serve as an accurate measurement of the background reflectivity. By directly measuring the background reflectivity one is able to adjust the fluorescence intensity in order to bring out the high contrast ratio required for reliable detection of the bar code. This approach, however, is limited by the degree of reflectivity of the background, and also requires a complicated reader system.
U.S. Pat. No. 5,418,855 entitled “Authentication System and Method” describes a process that contemplates use of fluorescent materials for authentication of articles through the use of invisible bar codes or other data forms. This patent describes improving imaging reliability through discrimination for wavelengths of fluorescent emission lines. However, in some cases fluorescent inks may fail to achieve total absorption of an excitation source. Furthermore, fluorescence from the ink found in the graphical images beneath or surrounding the bar code may also be detected, thus resulting in significant spatial modulation of the signals required for detection of a code or mark.
It is known that some materials formed of polymers, or mixtures of polymers, may be characterized by a lower critical solution temperature (LCST) below which a layer of the material is transparent or substantially transparent. Material that may be considered a “LCST material” may be either a simple polymer solution, or a mixture of mutually compatible polymers. Once heated above the LCST, an optical change takes place in the LCST material causing the layer to become opaque and thus visible. The use of LCST materials is known in the art, as evidenced by U.S. Pat. No. 4,722,595 entitled “Process for Displaying Optically Readable Information.” Although this patent teaches the use of LCST material for coding articles, this patent does not discuss or appreciate the problems that arise when an indicia imaging or reading system encounters a low contrast between the indicia and a substrate.
The use of LCST materials to record bar codes is also known, as evidenced by U.S. Pat. No. 5,298,476 entitled “Rewritable Bar Code Display Medium, and Image Display Method and Image Display Apparatus Using Same”.
Although the use of coding schemes has provided great value for certain applications, the coding schemes have not satisfied certain needs. That is, while invisible coding schemes have preserved desired visibility of important information, present systems using invisible coding schemes have failed to operate with a high degree of reliability where colored, multi-colored or visually complex backgrounds are present.
During the sorting and routing of flat mail such as plastic wrapped magazines and brochures it may become necessary to add additional information to the items. An invisible marking system is preferred in order to not obscure any information on the item. The highly colored and detailed designs of these items pose a significant problem for use with invisible bar codes.